1. Field of the Invention
This invention relates to novel substitutional solid-solution type crystalline oxides which are useful as a catalyst for vapor phase oxidation, particularly for producing maleic anhydride from a hydrocarbon containing four carbon atoms, and novel precursory substitutional solid-solution type crystalline oxides therefor.
2. Description of the Prior Art
For the preparation of maleic anhydride by the vapor phase oxidation of a hydrocarbon containing four carbon atoms, there are known catalysts which are composed of vanadium-phosphorus composite oxides as disclosed, for example, in Japanese Patent Publication No. 39037/1978, and Japanese patent application Laid-Open Specification Nos. 63982/1973, 95990/1976, 156193/1977 and 146992/1978, and which are composed of oxides containing vanadium, phosphorus, and iron, chromium, or the like as disclosed, for example, in U.S. Pat. No. 3,156,705, Japanese Patent Publication Nos. 43928/1978 and 43929/1978, and Japanese patent application Laid-Open Specification Nos. 60391/1978 and 61587/1978. The former group of known catalysts are composed of crystalline oxides, while the latter catalysts are amorphous.
In order to improve the performance of the vanadium-phosphorus composite oxide catalyst in vapor phase oxidation, I have made an extensive study of the suitability for vapor phase oxidation of a catalyst composed of an oxide containing not only vanadium and phosphorus, but also a third component. As the result, I have discovered that the addition of iron and chromium is detrimental to a crystalline vanadium-phosphorus composite oxide, though it may be effective to some extent to incorporate them into an amorphous vanadium-phosphorus composite oxide, but that a novel substitutional solid-solution type crystalline oxide containing iron, chromium and/or aluminum substituted for a part of vanadium, while retaining the crystal structure, of vanadyl pyrophosphate shown as (VO).sub.2 P.sub.2 O.sub.7, provides high activity and selectivity as a catalyst for vapor phase oxidation. I have also discovered that a novel substitutional solid-solution type crystalline oxide containing iron, chromium and/or aluminum substituted for a part of vanadium in a crystalline vanadium-phosphorus oxide having a specific crystal structure, while retaining its crystal structure, provides a precursor of the aforementioned substitutional solid-solution type crystalline oxide.